Under non-denaturing solvent conditions, the mean charge state of a multiply charged protein ion formed by electrospray is linearly correlated with the macromolecular surface

The charge states of protein ion species generated by electrospray under non-denaturing solvent conditions are strongly dependent on the occurrence of gas phase proton transfer reactions. Thus, by adding basic compounds to an array of model protein solutions, the charged states of multiply charged ions decrease with increasing the gas phase basicity of these additives. The role played by the basic (lysine and arginine) and acidic (aspartic and glutamic acids) amino acid side chains toward the proton exchange processes has been examined by using a series of basic compounds added to the protein solutions. In the present study, no relationship could be established between the presence at the protein surface of basic or acidic residues and the measured charged states. Actually, independently on their amino acid composition, the protein ions show a linear correlation between their mean charge state and their surface considered as a spherical area. (C) 2004 Elsevier B.V. All rights reserved.